A transfer system in a numerical control (NC) machine tool converts rotating force from a servomotor or other rotation driving sources into linear motion by means of a power transmission mechanism including a rack and a pinion, a ground ball-nut lead screw, or the like, and moves a work table for fixing a workpiece, tools for processing a workpiece, or the like to a predetermined position. Recently, in order to process a three-dimensional free surface of a workpiece, a numerical control (NC) machine tool equipped with a plurality of control shafts including five shafts is used.
In general, a decelerator, a ball screw, and the like, which are used in the numerical control machine tool, have a structure for removing a mechanical backlash by applying a pre-load, but when the motor is reversed in order to transfer the workpiece in the opposite direction after the workpiece is processed while being transferred in one direction, a phenomenon occurs in which the workpiece may not be immediately transferred in the opposite direction but slightly delayed. Because there is a problem in that processing precision deteriorates due to a backlash according to lost motion occurring when the motor is reversed, as described above, it is necessary to correct the backlash.
The backlash according to the lost motion occurs by a wound up or twisted phenomenon of a ball screw and a coupling due to a mutual relationship between stiffness and frictional force of the transfer system, rather than by mechanical hysteresis between a decelerator or a ball screw and a nut. That is, when the motor is reversed, the lost motion occurs due to a phenomenon that the workpiece does not immediately reach the instructed position, but only the ball screw and the coupling are wound up or twisted. The lost motion is increased as frictional force, which occurs by a relative motion with respect to a guide surface in the transfer system of the machine tool, and frictional force at the ball-nut lead screw become larger, and tends to be increased as the workpiece is positioned at a position far from the motor.
In recent years, the pre-load of the ball screw-nut is increased in some cases in order to raise the stiffness of a rotating shaft of the transfer system in a situation in which higher precision and stiffness of the machine tool is required, and in this case, the lost motion is inevitably increased due to the friction, and the wound up or twisted phenomenon because of the increased pre-load.
In order to solve the problem, in some cases, a manufacturer producing the machine tool provides a user with equipment in a state in which a backlash of the equipment is measured in a non-load state and the measured backlash is stored in a numerical control (NC) parameter, but in this case, there is a problem in that the backlash is varied as a usage condition of the delivered equipment is varied such that a correction thereof is not accurately performed. That is, there is a problem in that when a backlash amount is changed due to an influence caused by a change in a weight and a lubrication condition of the workpiece, a coefficient of friction of the transfer table of the transfer system, and the like, the user may not appropriately cope with the change in the backlash amount. Therefore, the user adjusted the change in the backlash by measuring the backlash again and correcting a correction value whenever the backlash is changed as a usage environment of the equipment is changed, which is very inconvenient.
Recently, a method of correcting the backlash is performed by measuring in advance the backlash of the workpiece for each weight, recording the measured backlash in the form of a look-up table in a non-volatile memory region of the numerical control (NC) apparatus, and allowing the user to input a corresponding processing code (for example, M code) after estimating a weight of the workpiece. However, in general, because the backlash of the equipment is not changed only by the weight of the workpiece, but other various variables affects the backlash, and there may be large deviations between equipment even though the workpieces having the same weight are used, there is an inconvenience in that the backlash of the workpiece for each weight needs to be measured and managed for each equipment. In addition, although the same equipment is used, the user may not appropriately adjust the problem when a backlash is changed while the usage environment of the equipment is changed.
To solve the above problem, a linear scale may be used as a manner of controlling a position by feeding back a position signal from a positioning detection device to a servomotor. The linear scale is a type of full-closed feedback system, and has high precision because the linear scale may directly detect the position. However, because the linear scale or the like requires an additional positioning detection device, there is a problem in that the linear motor or the like causes an increase in costs of the machine tool.
A method of indirectly controlling a position of a work table or an object to be position-controlled may be used by installing a rotation position detector such as a resolve or an optical rotary encoder in the servomotor, and feeding back an amount of rotation detected from the rotation position detector to the servomotor, thereby controlling the amount of rotation of the servomotor. The rotary encoder method is a type of semi-closed feedback system, and has a merit in that an additional positioning detection device is not required, but there is a problem in that it is difficult to perform an accurate position control with respect to the transfer table on which the workpiece is fixed.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.